Abstract: A structured-light visual sensing system for seam tracing was built on the arc welding robot, the extract arithmetic of trace path and the realization strategy for seam tracing were mainly researched.The control mode was adopted that sensor guided robot movement.In order to overcome the effect by look-ahead distance, the extracted tracing points were not used immediately, but stored in the circle queue, and they would be used when welding torch was nearby.The calculated driving vector made the welding torch move to the centre point of the welding seam.The torch would move forward at the given step length along the seam all the time, so the autonomous trace was implemented.The validating experiment on tracing S-curve seams by adjusting position and angle of torch proved that the developed system is robust, and the tracing process is stable and accurate.